In recent years, electroluminescent devices have been attracting attention as, for example, large-area solid-state light sources to replace incandescent lamps and gas-filled lamps. They have also been attracting attention as self-luminous displays, and are the most promising alternative to liquid crystal displays in the flat panel display (FPD) field. In particular, an organic electroluminescent (EL) device, in which the device material is formed from an organic material, is being commercialized as a low power consumption full-color FPD. Above all, polymer-based organic EL devices will be indispensable for future large-screen organic EL displays since the organic material of the polymer-based organic EL devices is formed from a polymer material for which film formation by printing, ink-jet, etc. is simple compared with low molecular weight-based organic EL devices, which require film formation in a vacuum system.
Conventionally, polymer-based organic EL devices employ either a conjugated polymer such as poly(p-phenylene-vinylene) (see e.g. International Publication WO90/13148) or a non-conjugated polymer (see e.g. I. Sokolik, et al., J. Appl. Phys. 1993. 74, 3584) as the polymer material. However, their luminescence lifetime when used in a device is short, which gives rise to problems when constructing a full-color display.
With the object of solving these problems, polymer-based organic EL devices employing various types of polyfluorene-based and poly(p-phenylene)-based conjugated polymers have been proposed in recent years, but they are not satisfactory in terms of stability.
As one means for solving this problem, a device utilizing phosphorescence from an excited triplet has been investigated. If phosphorescence from an excited triplet can be utilized, it can be expected that in principle the luminescence quantum yield would be at least three times that obtained when fluorescence from an excited singlet is utilized. Furthermore, while taking into consideration utilization of an exciton resulting from intersystem crossing from the singlet, which has high energy, to the triplet, it can be expected that in principle the luminescence quantum yield would be four times greater, that is, it would be 100%.
Examples of research that has been carried out so far include M. A. Baldo et al., Appl. Phys. Left. 1999, 75, 4. In this publication, the materials below are used. The materials are abbreviated as follows.
Alq3: an aluminum-quinolinol complex (tris(8-quinolinolato)aluminum)
α-NPD: N,N′-Di-naphthalen-1-yl-N,N′-diphenyl-biphenyl-4,4′-diamine
CBP: 4,4′-N,N′-dicarbazole-biphenyl
BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
Ir(ppy)3: iridium-phenylpyridine complex (tris(2-phenylpyridine)iridium)
Examples in which luminescence from a triplet is utilized include Japanese Patent Application Laid-open Nos. 11-329739, 11-256148, and 8-319482.